The present invention relates to a bias-voltage generator. More particularly, it relates to a bias-voltage generator suitable for measuring, with a high degree of accuracy, a leakage current.
A bias-voltage generator functions to supply a reverse bias voltage to an integrated semiconductor circuit substrate. Generally, an integrated semiconductor circuit contains in its substrate a great number of semiconductor devices. In such an integrated semiconductor circuit, the bias-voltage generator cooperates advantageously so that, first, the operational characteristics of these devices are improved and, second, P-N junctions created between the substrate and the respective diffusion layers formed therein are prevented from being forward biased. Such reverse bias voltage has conventionally been supplied to the substrate from an external bias-voltage supply located outside the substrate, but recently the tendency has been to form a bias-voltage generator inside the substrate as one body or unit with the semiconductor devices.
However, this forming of a bias-voltage generator inside instead of outside the substrate creates a problem when the integrated semiconductor circuit is probe tested in the usual manner, especially when the substrate leakage current is to be measured. The substrate leakage current is a current flowing from the power source to the substrate through any of the P-N junctions formed in the substrate. During the probing test, the level of the substrate leakage current is measured. It is then determined whether or not the level of the substrate leakage current is within a predetermined range.
Generally, when the substrate leakage current is measured, a current which is not such leakage current flows due to the presence of the transistors comprising the bias-voltage generator. If such a current exists, the substrate leakage current cannot be measured with a high degree of accuracy. Consequently, it is necessary to stop the current flowing through the transistors of the bias-voltage generator. The threshold level voltage of MOS (metal oxide semiconductor) transistors should be as low as possible so as to increase the operational capability of the bias-voltage generator (explained in detail hereinafter). However, the lower the threshold level voltage of the MOS transistors, the more effectively the MOS transistors operate in a so-called tailing region (explained detail hereinafter). However, if the MOS transistors operate in such a tailing region the current which is not the substrate leakage current is unnecessarily added thereto. Herein lies a contradiction.